LTREB Renewal: Social dynamics and fitness in a complex primate society

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Social species live in groups that vary dramatically in size. For any species, the maximum group size represents a breaking point, where the costs of group-living outweigh the benefits. Yet, most research on the social dynamics of groups has focused on societies with one level of social organization. These "single-level societies" are bound to one compromise solution when faced with competing selective pressures - grow in size to protect against predation? Or, shrink in size to alleviate feeding competition? Yet, a few societies have solved this group-size conundrum through the evolution of "multi-level societies" - comprising many modular core groups where individuals within the core group are consistent while the associations between core groups can quickly change. This research will explore how individuals transitioned from single- to multi-level societies by investigating the multilevel society of a wild primate, the gelada. This project asks two questions that each focus on a hallmark of gelada society. First, what permanently splits groups apart? And, second, what keeps groups together - forming the higher levels? This project has previously demonstrated that geladas have an unusually high tolerance for strangers. Identifying the mechanisms behind this remarkable tolerance could help us understand the evolution of another species' tolerance for strangers - humans. The directors of this project will continue a long-standing tradition of national and international STEM outreach (http://geladaresearch.org/db) and significant time investment into conservation efforts in Ethiopia through Save the Simiens, a 501(c)(3) non-profit they created dedicated to protecting the wildlife of the Simien Mountains National Park of Ethiopia where the research takes place. This project is co-funded by the Animal Behavior Program in the Directorate for Biological Sciences and the Biological Anthropology Program in the Directorate for Social, Behavioral and Economic Sciences.The objective of this research is to identify how individual interactions lead to the higher-order structure of multi-level societies by harnessing long-term data on multiple core groups of wild geladas (Theropithecus gelada), an easy-to-observe primate with a multilevel society. Specifically, this project will identify: (1) what causes core groups to split up (i.e., fission into two or more core groups) and (2) what causes multiple core groups to remain together (i.e., maintain higher-level cohesion). This research will investigate the fitness consequences of behaviors that produce a multi-level society and examine the mechanistic processes by which group structure may emerge from individual interactions. To understand why groups fission, it is necessary to track network dynamics to examine female social networks before and after a fission to identify the changes in social relationships that may predict an impending fission. To understand higher-level cohesion, this research will use GPS collars to track simultaneous movement across core groups that have (and have not) undergone fission. Thus, these results will identify the "decay" in association patterns across time. By identifying the origin of two key features of multilevel societies - small core groups and cohesion among those groups - this research will uncover not just why gelada units fission but how they fission; and this evolutionary pathway can then be tested in other taxa.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.